The longterm goals of the project are to identify the full array of efflux pumps of Staphylococcus aureus that contribute to multiple antimicrobial resistance and to elucidate the determinants of their expression, their role in microbial physiology and their effect on bacterial response to antimicrobials in infection. The work will focus on genetic analysis of regulatory elements and on bacterial fitness and response to antimicrobials in a subcutaneous abscess model, collaborating with other project groups to assess the efficacy of novel antimicrobial compounds in abscesses and the extent to which efflux pumps affect that efficacy. There are four specific aims: 1) assess the effects of NorB, NorD, and Tet38 efflux pumps and their regulators on response to antimicrobials in animal models of infection;2) evaluate the regulation of expression of abcA encoding an ABC family efflux pump and assess its effects on membrane and cell wall-targeting agents;3) analyze the global array of pumps over expressed in an abscess environment and determine their contribution to resistance to established agents and novel compounds;and 4) test novel compounds discovered in other subprojects of the program project for resistance to efflux pump expression and for efficacy and development of resistance in mammalian infection models. The work will utilize genetic manipulation and allelic exchange in S. aureus, measurements of gene expression with RT-PCR, and established murine models of infection (subcutaneous abscess, renal abscess, lethality) utilizing a genomically defined strains of methicillin resistant and other S. aureus. The overall goal of the program project is to take a well-integrated, multidisciplinary approach to understanding antibiotic resistance development and transmission, and to integrate that effort with the search for compounds that compromise resistant pathogens, including methicillin-resistant S. aureus (MRSA), by inhibiting novel targets and pathways. This project will add to understanding of resistance mechanisms related to multidrug efflux pumps and provide strains for testing the effect of such pumps on novel compounds active against new targets and pathways. It will also utilize mammalian models of a common MRSA infection to test compound activity in vivo.